The three dimensional distributions on the ribosome of elongation and initiation factors, of large and small subunit proteins, and of specific regions of RNA, will be determined. Other functional sites such as the peptidyl transferase and the exit site of the nascent polypeptide chain will also be mapped. Subunits will be reacted with antibodies directed against specific ribosomal proteins, against factors or against specific regions of RNA and observed by electron microscopy. Three dimensional sites will be mapped by comparing images of subunits (with attached antibodies) in different orientations. Our objective will be to relate the three dimensional structure of the tibosome to its function during protein synthesis. The small subunit is responsible for recongnizing the initiation site of mRNA with the participation of initiation factors and fmet-tRNA, for binding aminoacyl rRNAs, for associating with the large subunit, and for regulating the translational fidelity of messenger reading. The large subunit binds the acceptor stem of aminoacyl tRNA's entering the A site; catalyzes peptidyl transfer,and participates in elongation and translocation. By relating the three dimensional distributions of ribosomal proteins, factors and regions of RNA's with known biochemical information, we will attempt to elucidate the structural aspects of the molecular events occurring during protein synthesis. Comparative studies of ribosome structure will also be pursued in order to relate common structural features of prokaryotic, eukaryotic, and organellar ribosomes to common functions in protein synthesis.